The coverage provided by a cellular network is an important characteristic of the cellular network. Building a cellular network for complete coverage requires changing or adding cells by building base stations, which are also referred to as eNodeBs in an LTE implementation. However, to improve coverage, it is difficult to determine the correct position, optimal transmission power and emission direction for the base stations. Depending on geographic topography and areas available for new base stations, it is often difficult to deploy a cellular network without any “coverage holes”. Furthermore, information about the existence of such coverage holes is hard to obtain in a systematic way, since conventional techniques, such as Minimization of Drive Tests (MDT), allow collecting coverage information only if a mobile terminal has measurements available and is in a state of connectivity with the network in order to report signal quality.
Conventional MDT functionalities collect measurement results in an idle mode of the mobile terminal, only when the mobile terminal is in a “camped normally” state of the Radio Resource Control (RRC) idle mode, as specified in document 3GPP TS 36.304, Release 11. The conventionally reported information is, however, not sufficient to discover all coverage problems experienced by the mobile terminal in the idle mode.
Moreover, conventional Radio Link Failure (RLF) reports triggered by a radio link failure, e.g., during a handover, only contain measurement information related to the serving cell before the failure occurred and information about the recovery cell after recovery from the radio link failure. The RLF reports do, however, not include any information related to the time the mobile terminal spends in the RRC idle mode after the failure trying to find a suitable cell.
Document EP 2 559 285 A1 describes in one embodiment a mobile terminal performing MDT measurements. When a radio link failure occurs, the mobile terminal adds MDT measurement information to the RLF report sent to the network. In another embodiment, the mobile terminal detects that no reporting channel is available for reporting MDT measurement information and stores the MDT measurement information. After detecting the existence of a reporting channel, the stored unreported MDT measurement information is reported to the cellular network.
Document CN 102 685 672 A describes a mobile terminal recording information about a radio link failure and geographic information when the radio link failure occurs. The recorded information is reported to the network side in the RLF report.
Document EP 2 360 962 A2 discloses a technique for performing MDT measurements, such as periodic downlink pilot measurements, in a mobile communication network. A user equipment (UE) receives configuration information from a network to perform logging of measurements in an idle mode. The configuration information includes a logging duration, as well as an activation and a deactivation condition for the MDT measurement. The UE then performs the MDT measurement based on the activation and deactivation condition. When the UE encounters coverage holes or failures, it reports to the network immediately after the situation improves.
Document WO 2012/047070 A2 discloses a method for collecting information using a Minimization of Drive Tests (MDT) in a wireless communication system by setting a log environment based on a log configuration request message. MDT can be performed in a periodic mode, an event triggered mode, or a combination thereof. MDT results are logged, and the created log is reported to a source base station. A method is disclosed for controlling MDT measurements when handover of the mobile device is performed.
Document EP 2 627 112 A1 provides a radio measurement collection method, wherein a radio-environment measurement is performed in an idle state on a target LTE frequency band, a frequency band for a non-LTE radio access technology (RAT) and/or an LTE frequency band other than the target frequency band. Depending on a threshold value, the measurements of the LTE frequency band and the non-LTE RAT can be logged individually or combined, together with location information.
Document WO 2014/048474 A1 discloses a method for enhanced network management in the case of a communication failure between an Element Management System and a super-ordinate Network Management System. Once a failure occurs, the Element Management System is switched over to forward event reports to a predefined peer Element Management System. In case the communication is reestablished, the usual functionality is restored and the re-routing of event reports to the peer Element Management System is stopped.